<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        let year = +prompt('请输入年份')
        if(year % 4 ==0 && year % 100 !== 0 || year % 400 ==0){
            alert('这是闰年')
        }
        else( 
            alert('这是平年')
    )
        </script>  
</body>
</html>